The invention relates to a method for final rinsing of goods to be cleaned in a dish washing machine, the final rinsing being effected with clean hot water supplied from a water heater.
In order to satisfy hygienic requirements, it is requested in some countries that the final rinsing shall take place at constant temperature with a predetermined volume of water for a predetermined period. As far as these three operational parameters for the final rinsing are concerned, the standards are different in different countries. Also wants regarding the operational parameters may vary from one user to the other.
The water heaters which today normally are mounted in dish washing machines for final rinsing with clean hot water as the last step in a dish washing cycle are, however, of a construction which does not allow final rinsing with the above-mentioned values of the parameter temperature, volume, and period, and thus do not satisfy the standards. The water heaters moreover are rather space demanding. They have a laying flow tank with a heating element mounted in the lower half of the flow tank which has the water inlet in one end wall and the hot water outlet in the other end wall. The water is kept at a sufficiently high temperature in the tank, and there is accordingly at the beginning of the final rinsing a certain amount of water at the required temperature available in the tank. When the final rinsing is started water is supplied under pressure at the water inlet from an existing water distribution system and the water supplied usually is water from a distribution system, which is at about 55xc2x0 C., but nevertheless will be called cold water herein in order to differentiate said water from the much warmer water which is supplied by the water heater and is called hot water. When water is supplied to the tank, the hot water in the tank will be expelled therefrom to nozzles in the dish washing machine in order to sprinkle the goods to be cleaned, but already after a short period the temperature of the water in the tank has decreased below the predetermined value. This is due to the fact that the water flowing into the tank at the temperature 55xc2x0 C., which replaces the delivered rinsing water, mixes with the rinsing water already heated in the tank.
In the method according to the invention this drawback of prior art water heaters is eliminated by the hot water being supplied from a water heater having an elongated standing flow tank which has a cold water inlet with a shut-off valve at the lower end and a hot water outlet at the upper end, and heating elements inside the flow tank in the lower part thereof for heating the water in the flow tank. This type of water heater is basically known as disclosed for example in U.S. Pat. No. 5,137,053.
In such a water heater there is obtained a quieter flow in the flow tank, the water already heated being xe2x80x9cliftedxe2x80x9d in the flow tank and is discharged in the upper end thereof with a distinct and limited boundary layer between the hotter water and the incoming colder water which quietly rises in the flow tank below the boundary layer.
In order that a predetermined amount of hot water at a predetermined temperature shall be supplied during the rinsing procedure, it is required, however, that the boundary layer is located at a predetermined level in the flow tank when the supply of cold water is shut off and the delivery of hot water thus will be interrupted, which presupposes that the flow through the flow tank always is the same if the rinsing procedure is time controlled, that is if the shut-off valve for the supply of cold water to the flow tank is kept open for a predetermined period which is always the same for delivery of hot water from the water heater. Since the pressure of the cold water varies i.e. either it is taken from a municipal water distribution system or from a private water supply it will, however, be unavoidable that the boundary layer at such time control of the cold water supply will be located below said predetermined level in the flow tank if the pressure of the cold water is lower than the pressure for which the period during which the shut-off valve is to be kept open has been calculated so that a too small amount of hot water will be delivered from the water heater, or that also the boundary layer and colder water located below said layer in the flow tank will be delivered from the upper end of the flow tank so that stipulated requirements regarding the temperature of the water for final rinsing cannot be fulfilled in case the pressure of the cold water supplied is higher than the value assumed for calculating the period during which the shut-off valve is to be kept open.
It is possible to overcome the disadvantages following variations of the pressure of the cold water by providing a pressure maintaining valve between the shut-off valve and the cold water inlet at the lower end of the flow tank but this means that there is included in the system a sensitive component which moreover includes additional costs which are not unimportant.
A method is provided for final rinsing in a dish washing machine with clean hot water from a water heater having an elongated standing flow tank which has a cold water inlet with a shut-off valve at the bottom and a hot water outlet at the top and a heating element in the lower part. The temperature of the water is sensed at a location above the lower end of the tank. The shut-off valve is opened for supply of cold water under pressure to the lower end of the tank. Hot water is delivered from the upper end of the tank. The period from opening of the shut-off valve to the time of sensing a temperature decrease in a boundary layer between hot and cold water at the above-mentioned location is measured and on the basis thereof the flow and the period for movement of the boundary layer from this location to a desired position in the tank is calculated.